Electrodynamic transducers for vibrations transmitted through solids



Nov. 17, 1959 H. wAcHHoLz 2,913,701

ELECTRODYNAMIC TRANSDUCERS FOR VIBRATIONS TRANSMITTED THROUGH soLmsFiled July 2, 1958 2 Sheets-Sheet 1 Nov. 17, 1959 H. wAcHHoLzELECTRODYNAMIC TRANSDUCERS FOR VIBRATIONS TRANSMITTED THROUGH SOLIDS 2Sheets-Sheet 2 Filed July 2. 1958 upward more/naaf Unite StatesELECTRODYNABIIC TRANSDUCERS FOR VIBRA- TIONS TRANSMHTED THROUGH SOLIDSHelmut Wachholz, Hannover, Germany, assignor to Prakla Gesellschaft frPraktische Lagerstttenforschung G.rn.h.ll., Hannover, Germany, a Germancom- PallY Application July 2, 1958, Serial No. 746,223

Claims priority, application Germany July 5, 1957 comme (Cierto- 17),V,Y

This invention relates to electrodynamic transducers for vibrationstransmitted through solids. Y

One example of the use of transducers according to the invention is asseismometers or geophones, as employed in the investigation of theearths crust near to the surface by the seismic refraction or reectionmethod.

Especially for carrying out refraction measurements, it is necessary toemploy seismometers or geophones whose oscillatory system has a very lownatural frequency of oscillation, generally of a few cycles persecond,because it is such frequencies which are measured at relativelylarge distances from the shot point owing to the ltering action of theearths strata. Thus the natural frequency of the seismometers employedmust be of this order of magnitude in order to obtain sufficientsensitivity. Difliculties are encountered in the construction of suchseismometers, but such difficulties are not of a fundamental naturebecause any natural frequency of oscillation can be obtained byincreasing the size of the vibrating mass or reducing the springrestoring force by the use of very weak springs. However, in order toconstruct geophones which can be used in the field, that is to say`moved frequently from place to place, the total weight of the geophonemust not exceed a certain amount,

and `it is impossible to operate geophones `having very weak springswith a large mass, because the instruments areoftensubjected to roughtreatment so that damage to relatively large vibratory masses havingweak suspension springs would be unavoidable.

It is"an object of the present invention to provide an electrodynamictransducer having a low frequency of natural vibration, but whichnevertheless is robust and not readily damaged when used in the eld.

This object is achieved in accordance with the invention by providingtwo magnetic systems mounted on either side of the vibratory systemwhich includes a third magnetic system, the first two magnetic systemsbeing of such polarity that one system exerts a repelling action on themagnetic system of the vibratory system, while the other system exertsan attractive force thereon, in such manner that a zone of constantmagnetic force is produced which is equal to the value of the force ofgravity, but'of opposite direction. Consequently, the vibratory systemcan be regarded as substantially free from gravity within this workingregion.

Preferably, for damping the oscillations the nonmagnetic parts of thevibratory system are made of a material having high electricalconductance, preferably copper or t aluminium.

One construction of electrodynamic transducer Vfor solid-transmittedsounds will now be described, by way of example only, with reference tothe accompanying drawings in which:

Figure 1 is `a sectional` side elevation of the transducer, and

Figure 2 shows a curve of movement plotted against power forthevibratory systemof the transducer.

Referring firstly to Figure l, it will be seen that the transducer,which in this case is designed particularly for use as a seismometer orgeophone, comprises a housing 1 containing two pot-magnets 2 and amagnetic system 3. The oscillatory system of the transducer is mountedon springs or so-called spider diaphragms 8 and 9, the system consistingof a body 5 which supports an oscillation coil 6 and a magnet 4, thecoil 6 being suspended in an annular air gap in the magnetic system 3.

Considering the construction in greater detail it will be seen that thehousing .1, which is cylindrical, is provided near one end with aninternal screwthread 15 into which a closure cap 16, provided with anexternal screwthread 17, Visrradapted to be screwed. The cap 16 isformed with an annular groove 18 in a part adjacent the end face 20 ofthe housing 1, which groove 18 serves to receive a sealing ring 19 whichbears against the face 20 when the cover 16 is lrmly tightened and thusseals the housing 1 at this point against the penetration of moisture ordirt. The substantially round cap 16 may be attened on two diametricallyopposite sides, so that two Vparallel faces are formed, to which aspanner can be applied for tightening the cap.

Two conical projections 22 provided with an internal screwthread 21 eachserve to receive one cylindrically headed screw 24 provided with anextension 23. The loops of a carrier strap (not shown) can be passedaround the said extensions 23. The projections 22 are arranged in thelower third of the housing 1, so that when the transducer is gripped bythe carrier strap it is rotated through 180 from the operative positionshown in Figure l, whereby the vibratory system is restrained frommovement as will hereinafter be described. The housing base 26 situatedat the opposite end of the housing 1 to the cap 16 has a central bore 27accessible through an internally screwthreaded bore 28. The bore 28 isadapted to be closed by an externally screwthreaded disc 29 havingapertures 30 for receiving a Spanner. The disc 29 bears on the shoulders31 formed in the base 26 by the differing diameters of the bores 27 and28. The bore 27 serves for making the electrical connections between thetransducer and the measuring conductor. The measuring conductor can beintroduced from the side into the aperture 27 through a radiallyextending aperture 32. A device in the form of a spring wire spiral forprotecting the measuring conductor against bending is adapted to bescrewed from the outside of the housing into a screwthread cut in thesaid bore 32.

A further bore 33 of conical form, located eccentrically in the base 26is provided for the observation of a box spirit level 34 adapted to bepushed into a cylindrical bore 39 formed in the housing base 26concentrically with the bore 33. Annular grooves 35 and 36 extendingconcentrically around the bores 27 and 33 respectively in the base 26serve to receive sealing rings 37 and 38 which, in combination withparts of the actual transducer mounted in the housing 1, preventpenetration of moisture and dirt into the housing 1 through the base 26.

The vactual transducer mounted in the housing l consistsof three maincomponent elements: the main magnetic system 3, the vibratoryl system 5,and the additional magnetic system 2.

a circular pole face 45 which is substantially U-shaped' incross-section. A countersunk hole 46 is formed in the pole face 45 toconform to the shape of the head of the screw 42 which engages withascrewthread 4 7 provided I in the pot magnet 43. The magnet 3 istightly secured by thersaidscrew 42betweenthe recess44-V andV the rpoletace. 45. Rubber cushions 48V (only one. ofv which canV be seen Vin Fig.l) let into :the upper face of the pole face 45 damp the movement of thevibratory system 5 Whenthetflatter .approacheshepole `face45z .The side'wall 49, ofV hollowcylindrical form,tof the pot. magnet 43 surroundstheannular. magnet 3V andiits. pole face. 45.

Since. the pole face 45 and the pot mag-net 43` a'remarleV 'of highlypermeablematerial, the magnetic'eld produced by the'magnet: 3 extendsthrough the'- annular air gapiformedbylthe side faces. of the pole facet-and theV sidewall 49 Vof theV pot magnet143so that the'winding 6,

' hereinafterdescribed,y of the vibratory system 5I situated inaf'radial magnetic feldipf high held strength.

'l's'hefscrew 42V consistsv of' anon-magnetic material, forexamplefbrass, iV in t order ltoffavoid {shunting ofA the: mag'- neticeldlthrough'the Vscrew.1l2'. 'Thefpotgrnag'net 43' and consequently theentiremain, magnetic system` are securedbymeans of countersunk screwsSi) toa base ring 51slidably ttednthe housing 1. `V Alli-.the parts ofYthe vibratory system 4 are" mount onf a magnet support 52 constructed asa turned member ,Y withtthreel. external screwthreads 53,- 54, 55 .ofstepped. diameter. Screwed, on the screwthread- 53 is a turned.

cured to the coil for1ncr;support.56 by ,means of .thi-eef.

cuntersunk screws 57 (only one of which can be seen in Fig. l) andsupports the winding 6 inV aY recess formed inits periphery; Thearrangement isV such that when the transducer is.assembled the windingr6can move, in the aforesaid magnetic field in the airgap of the magnetsystem Y Disposed'in a corresponding recess in the upper face of thecoil former support V56 is a spring or spiderv 9,

which isV secured-against rotation byfcylindrical pins 6 1 driven intocorresponding ,bores in the coil former supportz'56 Yand projecting.VVintoyapertures in gthe spring 9 which.are in alignment with the.aforesaid boresg', The' spring yis clamped `between the magnet. cage 59and thej coil former` support 56 by screwing of the magnet cage 59ont'othe screw-thread 54 ofv the magnetsupport 52.

VSecureddrwithin the .magnetcagef 59`is the vibration rnagnet 4,V which.is held fast in the saidv cage 59' by a clamp,-

ingf nut 63 .screwed on the screw-thread 55 and' pressing onthe cover 60of the magnet;cage 59.

' tothe centre'of the cover '7,9 two glass bushes 82 extend throughtheoover with their upper ends' extendingV into Aspringor spider 8 tsagainstthe cover tlof thel magnetcage59 and is securedfa'gainst rotationby pins 62 (only one of whichcan be seen in Figure l), in a similarmanner toithe securing of Athe spring 9 to the isifalso screwed, presseson the nut 63 4and prevents iti-i voluntary loosening thereof. Aresilient ring 10 mounted..Y

spacing ring 69, which is disposed between the two springs 8,9 and theheight ofwhich` is equal to the distance between the two springs 8, 9 atthe point at which they are internally held. The spacing ring 69 is ofsubstantially trapezoidal cross-section. Its external diameter is sochosen that the ring is a sliding it in the housing 1.

Its `internal diameterV isV only slightly' greater than the externaldiameter of the magnet cage 59, so that the said i spacing ring 69 andthe magnet cage 59 simulta-v neously forma centering 'means for IVtheoscillatorysys'- tem 5 withinthe assembled transducer. In the course ofassembly, the spring 9is laid with its outerfedge on theV base ring'51and( the spacing .ring 69 is tightened byl means of screws 70 (only oneof which can be seen),

Vin themagnet'housing l72.;and in the spring 8, andk which'v tinto thescrewthreaded holes 74 in the .spacing ring 69;

The ,additionalmagnetic system 2 is mounted in the;` magnet housing 72.The actual annularvmagnets 2 lie' inan appropriately shaped-magnetholder 75 having a screwthread76 on its outer face.' vThe saidscrewthread iits into aninternal screwthread cutin the magnet'hous-- ingY7,2. Thus, the :distance between the magnets V2 and 4, and consequentlytheir -action uponione another, `can be varied by rotation of .themagnet holder`75 inV the magnethousi'1f1g72.v The adjustment ofthemagnet holder, 75 in the magnetihousing 72 Vislixed by a lock nut 77.Apertures 78 formed in the lock nut 77 serve to receive a screw-driver."'Y y v The actualtransducer is Vclosedat the top byI a cover.

79 which issecured to the magnethousing in such manf ner as to sealY itby'meansV of counter-sunk screws `80 (only` one of Ywhich can --be',seen) tting into corresponding l screwthreadedV holes 81 in the magnethousing 72. ,Near

the bore Y27=in the housing-,base 26.Y ,The lowerl terminalv portionsofA the glass bushes 82l are connected Vin elec-- trieally` conductivefashion toV the ,soldering tags 68, of the glass bushes 67 by means ofconnecting spirals 83 represented by crosses. The connecting spirals 83extend through central boresy ,84 andf85V formedrespectivelyim the locknut 77 and in the magnet holder 75. VThe terminals yofthe winding 6, arethuselectrically vconnected to the terminal portions (notshown) of theglass bushes 82, so that electrical leadsfrom an external indicatinginstru-v Y'ment ('notshownlmay be connected to the latter and therebyconnected .tojthe winding 6,.

.The'coverf579 is formed with a recess 86 at the pointcorrespondingto,thebore 39 in the housing base 26, and. Y

on the s aid lock nut64 serves asa cushion during transit,

as willi be describedhereinafter. Y

Formed axially withinA the magnet support 52 is'a lbore 65, kwhichmerges into two bores 66. eachv closed by a glass bush67. The saidbores'65, 66 serve to guideV the connecting-wires (not shown)` of thewinding, soy

that the beginning and the' end of thesaid Winding .arev electricallyaccessible through soldering tags, 68 projectingY from the lglassVbushes 67.

Allfthe parts, of the vibratory system 5--apar't'from` the magnet4--consistof materials whose relative per# meability constant is lin theneighbourhood of one.;v The the boxY spirit levelV 34 adapted to-besecured to the cover Y 79'by'means of screws' 8.7 (only one of-which isshown), caribe introduced into the said recess. .The base 'ring 51,thefspacing ring 1691themagnethousing 72 and' thecover y79 are4 of equalexternal diameterand areslidably fitted",

into the interior'ofthe'housing,L, *Centering devices 'con-Y sistingVoffcircula'r grooves havingcircular extensions 88,

8.9 engagingjtherein are provided on'the' three rst-n-ieri-V tionedparts to vcentre the interengaging parts. TheLexf g ternal diameterofthe pot magnet 43, whichissimilarly spring" 8, 9 consist, forexample;of spring bronze,fwhile thev remainingl parts consist mainly ofjcopperbrass or .u aluminiumk sothat thernagnetic eld produced by the:

magnetf 4 isformed as' in air. ,s

"Ihe springs-8, 9Ibearat-itheir outerf'peripherieson a Vcenteredwithithe base ring 51 bymeans of'a circular extension 90,`issinallerfthan the external diameter of the first-mentioned parts;V'Fitted over the pot maget43 isav i pressure ring `91 having' anexternaly screwthr'ead'tti'rig into the screwthread 15 formed internallyon Athe housingtl 1'. For the applicationsof ascr'e'w-'drive'n thepressure ring is Vftfnrnedlwith lanumberof recesses-(not shown-)ye Thepressure ring 91 bears at its pressure face against the free shoulder ofthe `base ring 51 and thereby forces the actual transducer fitted in thehousing 1 against the housing base 26 when the pressure ring 91 istightened in the screwthread 15. The packing rings 37 and 38 disposedwithin the housing base 26 thus bear against the cover 79 and againstthe spirit level 34 respectively'and thereby form aseal. An example ofthe polarity of the magnets 2, 3 and 4 is indicated in .Figure l, fromwhich it will be seen that the pole of the pot-magnets 2 which isnearest to the adjacent pole of the movable'magnet 4 is of 0ppositepolarity to that pole of the magnet 4, whilst the other pole of themovable magnet 4 and the adjacent pole of the magnet 3 are oflikepolarity. In this way the magnet 4 is situated in a uniform magneticfield provided by the magnets 2 and 3. When the transducer isverticallyV mounted, the whole mass of the oscillatory system is drawndownwards by the earths attraction. The magnet 4, however, is bothrepelled upwards by the lower magnetic system 3 and attracted upwards bythe other magnet 2. In accordance with the invention, the three forcesare arranged to be substantially in equilibrium, so that the static massof the oscillatory system can be regarded as substantially zero. Thismeans that when externally acting forces are applied to the base of thetransducer housing 1, only the dynamic action of the oscillatory mass iseffective, and the natural frequency of oscillation of the oscillatorysystem can be brought to as low a frequency as a few cycles per secondalthough using springs 8 and 9 having a spring stiffness which canreadily be achieved in practice. It will be appreciated that thecompensating action of the magnetic elds is maintained even withsubstantial movements of the oscillatory system, due to the fact thatwhen the oscillatory system moves upwards the distance between magnets 4and 3 increases and that between magnets 4 and 2 decreases. Therefore,the repelling force of magnet 3 on magnet 4 is reduced, but theattractive force of magnets 2 on magnet 4 increases to the same extent,so that the total action of the magnetic forces remains the same and theeffect of the earths gravitational field on the oscillatory system isfully compensated in all movements.

Figure 2 shows a force-movement diagram for the oscillatory system. Theforce or power acting on the oscillatory system in each instance isplotted along the ordinates. The corresponding location of theoscillatory system is designated as a movement in the diagram becausethe oscillatory system carries out a movement under the influence ofground disturbances. The upward movement is plotted along the abscissae.

The oscillatory system is subjected to the action, on the one hand, ofthe earths attraction, which is represented by the broken line 130. Inaddition, the repelling force of the magnet 3 acts on the oscillatorysystem. This latter force is represented by the curve 131 and is greatlydependent upon the amount of movement. It is oppositely directed to theearths attraction. The force of the attracting magnets 2, represented bythe curve 132, acts similarly but in laterally inverted fashion.

The forces illustrated by the curves 131 and 132 add together to formthe curve 133. This curve is substantially linear within the workingregion and opposite to the force of gravity. By appropriate design it isarranged that the force represented by the curve 133 over the workingrange is made equal in magnitude to the force of gravity. Within theworking region, therefore, an approximately complete cancellation of theforce of gravity is obtained and the weight of the vibratory system isthus eliminated.

When the transducer is exposed to the earths vibrations, the oscillatorysystem 5 remains substantially stationary owing to its inertia, and theremaining parts of the transducer then oscillate about the oscillatorysystem 5 in sympathy with the earths vibrations. The magnetic field,which extends radially from the pole face 45 to the sidewall 49 throughthe winding 6 disposed in the gap defined by'l the two, is thereby movedrelatively to the said winding 6. The individual turns of the winding 6thusv intersect magnetic field lines, whereby an electromotive force ewg-(I.- da

is induced in the winding 6, which is applied in the described manner tothe measuring leads by which it is conducted to a suitable indicatinginstrument for giving an'indication representative ofthe earthsvibrations.

For damping the natural frequency and producing a uniform lfrequencyresponse curve, it is desirable to make the entire coil body and magnetholder 5 of a material of high electrical conductance, for examplecopper, brass or aluminium. In this way, a uniform response curve from afew cycles per second can be obtained Without diiiiculty.

It is not necessary to provide any device for locking the oscillatorysystem during transport of the transducer, instead the transducer isinverted so that the resultant magnetic force is additive to the forceof gravity instead of being in opposition thereto. As a result, theoscillatory system is brought to rest by engagement of the resilientring 10 against the surface 11 of the magnet holder 75.

I claim:

l. An electrodynamic transducer comprising a housing, a first magneticsystem fixed in said housing, said first magnetic system having an airgap therein, a movable electric coil located in said air gap, resilientsuspension means, said resilient suspension means being secured to saidhousing and to said moving coil for resilient suspension from saidhousing of said coil in said air gap, a second magnetic system, saidsecond magnetic system having two opposed pole faces one of which facestowards said first magnetic system, said second magnetic system beingsecured to said moving coil and forming an oscillatory system therewith,a third magnetic system, said third magnetic system being secured tosaid housing opposite that face of said second magnetic system which isremote from said first magnetic system, the polarity of one of the polefaces of said second magnetic system being the same as that of themagnetic field producedV at that face by the magnetic system oppositethat pole face, Whilst the polarity of the other pole face of `thesecond magnetic system is of opposite polarity to the field produced bythe magnetic system opposite that pole face, `the resultant magneticforce acting upon said second magnetic system being equal in magnitudeto the gravimetric force acting upon said oscillatory system andopposite in direction to said gravimetric force when said transducer ispositioned in its operational position.

2. An electrodynamic transducer comprising a housing, a first magneticsystem fixed in said housing, said first magnetic system having an airgap therein, an oscillatory system, said oscillatory system consistingof an electric coil, a coil support and a permanent magnet, saidelectric coil being located in said air gap, resilient means, saidresilient means being secured to said housing and to said coil supportfor resilient suspension of said coil in said air gap, said permanentmagnet having two opposed pole faces one of which faces towards saidfirst magnetic system, a further magnetic system, said further magneticsystem being secured to said housing opposite that face of saidpermanent magnet which is remote from said first magnetic system, thepolarity of one of the pole faces of said permanent magnet being thesame as that of the magnetic field produced at that face by the magneticsystem opposite that pole face, whilst the polarity of the other poleface of said permanent magnet is of opposite polarity to the fieldproduced by the magnetic system opposite that pole face, the resultantmagnetic force acting upon said permanent magnet being equal inmagnitude to the gravimetric force acting `upon-said oscillatorysystem-andn oppositerin'direction toA said glravimetri'c-V forcewhenhsaid transducer is positioned in its operational position. f

34. A transducrerriaccording to claim V2Q whereinsaid coil Y support iscomposed of a metal of highY electricalV conductance.

4. A transducer accordingY to: claim'r, wherein said metal is copper.

5. A transducer according to Vclaim-'Siglwherein said' metalis'aluminium. Y 6,; An electrodynamc transducer comprising ahousing, arstmagnetic systemlxed inthe lowerfendV ofV said housing, said rstmagnetic system having 'an airl gapY i therein, an oscillatory support,anelectric coil, said coil Vbeing located in said air gap andnsecured tosaid oscil- Y latory support, ai permanent magnet secured to'saidoscillatory suppormeans for supporting said oscillatoryisupportwithinsaid housing above said'vfrrst magnetic'system, said permanentmagnet having two opposed poletfaces the lower one-of which-,facestowards 'said first` magnetic?V system, va further magnetic;system s,aidfurthern magnetic magnet being'equal in m'agnitll'de t0 the' acting-upon said oscillatory system..-. t

y- 1 References Cited-.in thetile .of thispatentv 'A I UNITED STATES'PATENTS*

